II · THE DISCOVERY · HISTORY OF SCIENCE
Steel
Steel is iron with carbon mixed in — rarely more than two per cent. That small addition changes how the metal behaves under stress, and how it can be shaped.
At a glance
- Composition
- Iron alloyed with carbon, typically under 2% carbon content
- Discovery date
- 1865
- Fabrication method
- Steelmaking
- Key property
- Improved mechanical properties compared to pure iron
Look closer
The carbon content is small but decisive
Steel is defined as an alloy of iron and carbon with a carbon content not far exceeding two per cent. That narrow range matters: the carbon atoms sit within the iron lattice — the repeating three-dimensional arrangement of iron atoms — and inhibit deformation under mechanical stress. The strengthening is primarily due to this introduction of carbon into the iron structure. Add too much carbon and the material becomes brittle; too little and you have essentially iron. The useful zone is a thin one.
Other elements produce different grades
Iron and carbon are always the main elements in steel, but other elements are used to produce various grades demonstrating altered material, mechanical and microstructural properties. Stainless steels, for example, typically contain eighteen per cent chromium and exhibit improved corrosion and oxidation resistance compared to their carbon steel counterpart. Galvanised steel is coated in a layer of zinc to achieve a similar effect. The engineered mechanical properties are generally at the expense of the ductility — the ability to deform without breaking — and elongation of the pure iron state, which decrease upon the addition of carbon.
The microstructure depends on thermal history
Under atmospheric pressures, steels generally take on two crystalline forms: body-centred cubic and face-centred cubic. Depending on the thermal history — how the steel was heated and cooled — and alloying, the microstructure may contain the distorted martensite phase or the carbon-rich cementite phase, which are tetragonal and orthorhombic, respectively. These phases affect the mechanical properties. The behaviour of the finished material is not just a matter of what is in it, but of how it was made.
The story
Steel had been produced in bloomery furnaces for thousands of years before the date recorded here. A bloomery is a type of furnace in which iron ore is heated with charcoal, producing a spongy mass of iron mixed with slag — the waste material — which is then hammered to drive the slag out. The result is wrought iron, which contains very little carbon. Steel, with its higher carbon content and improved properties, could be made in small quantities by adding carbon to iron or by carefully controlling the process, but large-scale industrial use began only after more efficient production methods were devised.
In the seventeenth century, the introduction of the blast furnace and production of crucible steel — steel made by melting iron with carbon in a sealed clay pot — allowed more controlled manufacture. The Bessemer process, introduced in England in the mid-nineteenth century, marked a new era of mass-produced steel. The process involved blowing air through molten iron to remove impurities and excess carbon, producing steel far more quickly and cheaply than earlier methods. This was followed by the open-hearth furnace, another method of refining iron into steel.
The date recorded — 1865 — falls within this period of rapid development. Whether it marks the Bessemer process reaching a certain scale, a patent, or another milestone in steelmaking, the facts do not specify. What is clear is that the mid-nineteenth century saw the transformation of steel from a material made in small batches to one produced industrially.
Mild steel — steel with a low carbon content — replaced wrought iron in many applications. The German states were the major steel producers in Europe in the nineteenth century. American steel production was centred in Pittsburgh, Bethlehem, Pennsylvania, and Cleveland, Ohio until the late twentieth century. Further refinements in the process, such as basic oxygen steelmaking, largely replaced earlier methods by further lowering the cost of production and increasing the quality of the final product.
Why it mattered then
Before steel could be made cheaply and in quantity, construction and manufacturing were constrained by the materials available. Wrought iron could be worked and shaped, but it was softer and less strong than steel. Cast iron was hard but brittle. Steel combined strength with workability, but making it was slow and expensive. The Bessemer process and the developments around it changed that. Mild steel replaced wrought iron. Structures that had been built in wood or stone could now be built in steel. Railways, bridges, ships and machines could be made stronger and lighter. The scale of production increased sharply: what had been a craft material became an industrial one. The German states and then American centres built industries around it. The material itself was not new, but the ability to make it in large quantities, reliably and affordably, was.
Why it matters now
More than 1.6 billion tons of steel are produced annually. It is used in structures as concrete reinforcing rods or steel beams, in bridges, infrastructure, tools, ships, trains, cars, bicycles, machines, electrical appliances, furniture and weapons. World steel production is now centred in China, which produced fifty-four per cent of the world's steel in 2023. The modern steel industry is one of the world's manufacturing industries, but also one of the most energy and greenhouse gas emission intense industries, contributing eight per cent of global emissions. Steel is also very reusable: it is one of the world's most-recycled materials, with a recycling rate of over sixty per cent globally. The material is so widely used that its production and recycling are now measured as a proportion of global industrial activity and global emissions. What began as a method of improving iron has become a foundation of modern infrastructure.
The surprising detail
The mechanical properties that make steel useful — its strength, its resistance to deformation under stress — come at the expense of properties that pure iron has. Pure iron is more ductile, meaning it can be stretched or deformed more before it breaks. Adding carbon to make steel increases strength but decreases ductility and elongation. The engineered mechanical properties are generally at the expense of the ductility and elongation of the pure iron state, which decrease upon the addition of carbon. In most applications, that trade-off is worth making, but it is a trade-off nonetheless. The improved material is not improved in every respect; it is improved for the uses that matter.
What is disputed
The discovery date of 1865 is recorded in Wikidata, but the reference material does not specify what event or milestone that date marks. The Bessemer process was introduced in England in the mid-nineteenth century, and 1865 falls within that period, but whether the date refers to a patent, a particular installation, or another development in steelmaking is not stated in the sources. The material confirms that large-scale industrial use of steel began after more efficient production methods were devised in the seventeenth century and expanded significantly in the mid-nineteenth century, but the specific significance of 1865 is not explained.
Remember this
Steel is iron with a small amount of carbon added. That addition changes the mechanical properties, making the material stronger but less ductile.
Test yourself
Steel is stronger than pure iron, but what property does pure iron have more of?
Ductility and elongation. Pure iron can be stretched or deformed more before it breaks. Adding carbon to make steel increases strength and resistance to deformation under stress, but decreases ductility and elongation. The engineered mechanical properties are generally at the expense of the ductility and elongation of the pure iron state, which decrease upon the addition of carbon. In most applications, the increased strength is worth the loss of ductility, but it is a trade-off: the material is not improved in every respect, only in the respects that matter for the intended use.
Go deeper
Image: Andrea Pavanello , Milano, CC BY-SA 3.0 it. Licence: CC BY-SA 3.0 it. Source.